Method of making steel having an improved hardenability

ABSTRACT

A METHOD OF MAKING STEEL HAVING AN IMPROVED HARDENABILITY WHEREIN THE STEEL, PRIOR TO TEEMING, IS RAISED TO A TEMPERATURE OF NOT LESS THAN 1675*C. IF DESIRED,   ALUMINIUM AND/OR TITANIUM MAY BE ADDED TO THE MELT BEFORE OR DURING TEEMING.

United States Patent Int. Cl: C21c 7/10 US. CI. 75-49 9 Claims ABSTRACT OF THE DISCLOSURE A method of making steel having an improved hardenability wherein the steel melt, prior to teeming, is raised to a temperature of not less than 1675" C. If desired, aluminium and/or titanium may be added to the melt before or during teeming.

BACKGROUND OF THE INVENTION (1) Field of the invention This invention relates to a method of making steel. The hardenability of a steel essentially depends upon the transformations which ta'ke place during cooling. The less transformation there is to non-martensitic transformation products such as pearlite or bainite then the greater is the hardness of the steel.

Steels are hardened by quenching at a sufficiently fast rate to prevent transformation to phases other than martensite. However, the hardenability of a steel is limited by the maximum rate at which the steel can be quenched and this is dependent upon the rate at which heat can be lost from the interior of the steel through the surface of the steel.

(2) Description of the prior art Thus, to increase the hardenability of steel, alloying elements are added, which retard the transformations during quenching.

Thus, a plain carbon steel such as, for example, EN15 having few alloying elements has less inherent hardenability than a more alloyed steel such as, for example, EN24 which has more alloying elements present to retard the transformation. However, a plain carbon steel is more economical to make than an alloying steel.

SUMMARY OF THE INVENTION An object of the present invention is to provide a steel having good hardenability and which is economical to produce.

According to one aspect of the invention we provide a method of making a steel having an improved hardenability including the steps of heating a hardenable steel melt, prior to teeming, so that the Whole of the melt is at a temperature of not less than 1675 C. ascertaining that the whole of melt is at a temperature of not less than 1675 C. and maintaining the whole of the melt at a temperature of not less than 1675 C. for a predetermined time lying approximately in the range of 2 minutes to 30 minutes, adding to the melt a hardening element selected from the group comprising aluminium, titanium and, aluminium and titanium to achieve a final melt content of between 0.03% and 0.10% aluminium, and about Patented Sept. 12, 1972 0.03% titanium, and between 0.03% to 0.10% aluminium and about 0.03% titanium.

By a steel of improved hardenability" we mean a steel having an improved hardenability compared with a steel identical in composition, but which has not been subjected to heating to a temperature of at least 1675 C.

The aluminium and/ or titanium may be added whilst the metal is at a temperature of not less than 1675 C. or may be added after the metal has been allowed to cool from a temperature of not less than 1675 C. For example, if added during teeming, the temperature of the melt may normally lie in the range 1575 to 1600 C.

Alternatively, the method may include the further step of adding to the steel melt, prior to teeming, or to the metal stream during teeming, a substance or substances in such proportions as to render nitrogen or nitrides in the melt ineifective as nuclei for precipitation of non-martensitic transformation products during transformation of the steel.

the desired composition of the steel melt, is by arranging the composition to give a predetermined hardenability factor in accordance with the formula set out below. The composition of the melt is adjusted by adding elements, within the ranges mentioned in the preceding paragraph, in order to bring the hardenability factor, as determined by the formula set out below, to the desired value.

Thus, by making additions in accordance with the formula, to give the same hardenability factor, a steel of t he same hardenability may be obtained even though the coinposition of the steel is different to that of another steel whose composition is arranged in accordance with the formula to give the same hardenability factor.

The formula used is:

Factor: 1.375 87+ 1.02692 silicon percent-P0490 25 X copper percent+0.5022 1 6 X manganese percent +2.3 8480 x carbon percent+().35 889 x chromium percent+ 1.07513 X molybdenum percent-0.05591 BS grain size number-2.46496 sulphur percent 0.48334 nickel percent+3.37670 phosphorus percent-0.59522 tin percent.

The aim is to produce a steel with a factor of not less than 0.80. The melt is made by roughly making up the charge using elements selected within the ranges specified in the paragraph above and then performing a rapid analysis on the melt, and then making adjustments to the composition so that the factor determined by the formula set out above is brought to the desired value.

One composition having a hardenability factor of 0.825 is as follows:

Approximately 0.25% copper and/or 0.030% tin may also be present.

DESCRIPTION OF THE PREFERRED EMBODIMENT One example of a method of making steel according to the invention will now be described in more detail. In this example a steel melt having the following composition was made by charging the furnace with a charge made up roughly to the desired composition by adding elements selected within the range specified in column 2. A rapid analysis of the melt was then performed and the hardness factor calculated, using the formula set out above. Necessary additions were then made in order to bring the hardenability factor to the desired hardenability factor of 0.80 min. and it was found that after the additions had been made a steel having a factor of 0.825 had been achieved.

Compositions of the melt was as follows:

In all other respects, the making of the melt followed conventional practice.

Immediately prior to teeming, the temperature of the melt was raised to a temperature of approximately 1700 C. and the temperature of the melt was ascertained, and the melt was maintained at approximately the above mentioned temperature for a sufficient time for the desired enhanced hardenability properties of the steel to be achieved. It has been found that in practice a time lying within the range 2 minutes to 30 minutes in suitable. If desired, however, the time may exceed 30 minutes without any deterioration in the hardenability. It has been found that approximately 2 minutes is the minimum time required.

It will be appreciated that the temperature throughout the whole of the melt is, in practice, not the same and will vary within the usual limits. It is important, however, that the whole of the steel melt is at a temperature above 1675 C.

Whilst the steel was at the increased temperature of approximately 1700" C. the aluminium was added to achieve a final melt content of 0.07% aluminium.

If desired, alternatively, or in addition, titanium may be added to give a final melt content of up to 0.03%.

Alternatively, the additions of aluminium and/or titanium may be made to the ladle or to the metal stream during teeming and the temperature of the metal may be allowed to fall below 1675" C. during the making of aluminimum and/or titanium additions to the metal during teeming.

It has been found in practice that if aluminium and titanium are added together slightly less of each is needed.

With the steel described above and with the treatment described above it has been found that a hardenability similar to that which would be expected from a steel of, for example, EN24 composition is achieved. It will be appreciated that the composition described above is basically similar to a steel of EN15 specification. Thus, a steel having only 0.20% nickel, 0.25% chromium and 0.1% molybdenum has a similar hardenability to a steel having 1.5% nickel, 1.25% chromium and 0.5% molybdenum. Thus, a steel which is basically a low alloy steel and hence which is relatively economical to produce, is provided with a hardenability substantially similar to that of a much higher alloy steel which would be more expensive to produce.

A jominy test was carried out on the following steels:

(1) The steel described above.

(2) A steel of the same composition and raised to the same temperature as described above but to which no aluminium and/ or titanium additions were made, and

(3) A steel of the same composition as the steel described above but which had not been raised to a temperature of at least 1675 C. and to which no aluminium and/ or titanium additions have been made. All were subjected to a jominy end quench test and the results are shown in the figure where the curves 1, 2 and 3 show the results obtained with the steels indicated at 1, 2 and 3 above respectively.

In the examples described above, the steels were all of the same composition but it will be appreciated that if desired the steels could have been of different composition but having their compositions determined by the formula set out above so that all the steels had the same hardenability factor. In this case the same results would have been obtained as were obtained in the examples shown in the figure.

Furthermore, usually alloy steels of the EN24 type have enhanced tempering resistance as well as enhanced hardenability compared with an EN15 steel. By performing the method of the present invention a relatively high degree of tempering resistance is also achieved in a basically EN15 type steel as well as the above described enhanced hardenability.

It is thought that the mechanism of the increased hardenability achieved by the present invention is as follows:

Normally in 'a steel melt nitrogen is present and is usually present in the form of nitrides. The nitrogen, either when present as a nitride or in some other form, can normally act as nuclei during cooling transformations of the steel and thus increase the rate of the transformation and so reduce the hardenability of the steel.

It is thought that by heating the melt to the increased temperature mentioned above the nitrogen is freed and when titanium or aluminium is added the titanium or aluminium complex the nitrogen and render it ineffective as nuclei.

It is thought that when no titanium or aluminium is added although most of the nitrogen again becomes effective as nuclei some is prevented and so the total amount of nitrogen available as nuclei is reduced after the subjection of the melt to the increased temperature.

Thus, although titanium and aluminium have been mentioned as additions it should be appreciated that any other material which is suitable for rendering the nitrogen ineffective as nuclei may be added.

Although treatment of an EN15 type steel has been described it is thought that the method of the present invention may be applied to a wide range of steel including medium alloy steels.

If desired, vacuum treatment of the melt may be performed to remove hydrogen because the presence of the relatively large amount of aluminium which can be present in the steel as a result of performance of the method according to the invention can give rise to ingot cracking due to hydrogen retention. The vacuum degassing may be performed in the ladle in conventional manner.

All percentages in this specification are percentages by weight.

The addition of aluminium to steel normally reduces the hardenability of the steel because of the grain refining effect of the aluminium. In the case of the present invention the grain size would be expected to reduce the hardenability slightly. However, it has been found that any such reduction in hardenability is more than offset by the effectiveness of the heating of the melt to a temperature of not less than 1675 C. plus addition of aluminium and/or titanium.

It has also been found that the presence of about 0.1% molybdenum is also beneficial for whilst it would be possible solely on the grounds of hardenability to omit the molybdenum and to increase the chromium content such a small amount of molybdenum is beneficial in combating temper imbrittlement in the steel.

Thus a steel having the specific melt composition described hereinbefore and subjected to the method of the present invention has an optimised performance in terms of hardenability, temper resistance and impact strength.

What is claimed is:

1. A method of making a steel having an improved hardenability relative to a steel of substantially the same formulation not subjected to the method hereinbelow comprising the steps of providing a hardenble steel melt, heating the steel melt prior to teeming so that the whole of the melt is at a temperature of not less than 1675 C., ascertaining that the whole of the melt is at a temperature of not less than 1675 C. and maintaining the whole of the melt at a temperature of not less than 1675 C. for a determined time lying approximately in the range of 2 minutes to 30 minutes, adding to the melt a hardening agent selected from the group consisting of aluminium to achieve a final melt content of between 0.03% and 0.10% aluminium, titanium to achieve a final melt content of about 0.03% titanium and aluminium and titanium to achieve a final melt content of between 0.03% and 0.10% aluminium and about 0.03% titanium and teeming the melt.

2. A method according to claim 1 wherein said agent is added while the whole of the melt is at said temperature of not less than 1675 C.

3. A method according to claim 1 wherein said agent is added after the melt has been allowed to cool from said temperature of not less than 1675" C.

4. A method according to claim 1 wherein the agent is added to the melt prior to teeming.

5. A method according to claim 1 wherein the agent is added to the metal being teemed.

6. A method according to claim 1 wherein said steel melt has the following composition:

7. A method according to claim 1 wherein the melt has the following composition:

Percent Carbon 0.42 Manganese 1.59 Silicon 0.43 Chromium 0.25 Nickel 0.22 Phosphorous 0.030 Sulphur 0.033 Molybdenum 0.1 1 Copper 0.17 Tm 0.026 Iron and unavoidable impurities Balance 8. A method according to claim 1 wherein the composition of the melt is in accordance with the following formula:

Factor: 1.375 87+ 1.02692X silicon percent+0.49025 X copper percent+0.50216 X manganese percent 2.3 8480 X carbon percent+0.35 889 X chromium percent+ 1.07513 X molybdenum percent-0.05591 X BS grain size number-246496 sulphur percent +0.48334 X nickel percent+ 3 .37670 X phosphorous percent-0.5 95 22 X tin percent wherein the factor is not less than 0.80%

9. A method according to claim 1 wherein the melt is vacuum treated to remove hydrogen.

References Cited UNITED STATES PATENTS 3,262,776 7/1966 Loefller et a1. 129 3,459,537 8/ 1969 Hornak 75--58 2,693,414 11/1954 Dunn et al 75-129 RICHARD O. DEAN, Primary Examiner US. Cl. X.R. 

